The invention relates to an electric furnace such as is known for example from the German Patent Specification No. 21 95 75.
Advances in the development of semiconductor components in previous years have brought about increasing usage of direct current arc furnaces in the iron and steel industry for the electric smelting of steel.
The construction and method of operation of direct current arc furnaces are known for example from the journal "Stahl und Eisen" ("Steel and Iron"), 103 (1983) No. 3, Feb. 14, 1983, pages 133 to 137.
In the case of direct current arc furnaces, in order to optimise the electrical and thermal relationships, it has been shown to be advantageous to form the arc between one or more electrodes, arranged above the melting charge, and the melting charge itself. At least one electrode, the bottom electrode, which is in the bottom of the furnace and is in contact with the melt, is provided for the return circuit of the direct current. The bottom electrode is exposed to a continuous and very high thermal stress, for which materials having a very high fusion and melting point, for example graphite, are suitable. On the one hand, however, when carbon electrodes are used the melt is carbonised. But this is not desirable, especially during the manufacture of low-carbon steels. On the other hand, the carbon electrodes are consumed, which can weaken the furnace bottom and unfavourably affect the electrical power transmission.
According to the proposal of German Patent Specification No. 21 95 75 for achieving the object therein, the bottom electrode consists of a cluster of iron bars which are connected at their lower end to a plate which is also made of iron. The electric current is supplied to the material to be melted or to the melting bath via the plate and the iron bars. A refractory dam which is firmly rammed down is located between the bars and all around the cluster of bars, which dam, in the present case, is made of a magnesite structural material.
A bath movement is brought about by the electromagnetic field of the current flowing through the melting bath from the bottom electrode to the top electrode, which bath movement is particularly intense at the melting bath contact surfaces of the bottom electrode, that is, at those transition areas where the electric current passes over from the relatively small cross section of the bottom electrode to the relatively large cross section of the melting bath.
The melting bath flow acts on the partial contact surfaces which then melt back under the effect of temperature slightly behind the hearth surface, which causes small indentations, so called craters, to form. As a consequence of the relatively high kinetic energy of the bath flow, a cross flow (secondary flow) is induced in these indentations. This causes the partial contact surfaces to melt down still further. However, melting down of the iron bars at their end facing towards the melting bath is to be avoided if possible or at least reduced to a harmless level, because the craters (local cavitation) are not only restricted to the iron bars but also affect the adjacent areas of the refractory structural material, so that crater-like recesses develop. When the liquid charge is poured out of the furnace, the craters are then like-wise emptied and hollow spaces develop which impede subsequent electrical contact of solid constituents to be melted. The intensity of the bath movement is of course also dependent on the strength of the electromagnetic field. For a predetermined current intensity, this electromagnetic field becomes weaker the longer the magnetic field lines are, that is, the greater the periphery or diameter of the bottom electrode is. Because the forces directed towards the melting bath act at right angles to the electromagnetic field lines, a bath movement forms at right angles towards the magnetic field lines, that is, from outside towards the axis of the bottom electrode.